Agriculture Reference
In-Depth Information
biochemical reactions and are an integral part of nutrient cycling and soil fertility.
Therefore, this chapter evaluated the impact of heavy metals on
Rhizobium
populations
isolated from a lead mine which activity ceased 50 years ago. In order to reach this goal
some physicochemical parameters that influence metal bioavailability was determined, as
well as metal concentrations in soils. Soil enzyme activities are highly affected by soil
conditions and their evaluation may provide useful information on soil microbial activity
and survival. For this reason soil enzyme activities have been proposed as biological
indicators of pollution, specially organic, but information about the influence of heavy
metal on soil enzyme activities is scarcer. Thus, it was determined the activity of
enzymes such as dehydrogenases,, hydrolases, phosphatases, catalase and lipase in heavy
metal contaminated soils. Metal tolerance of
Rhizobium
isolates was also screened in
artificial media supplemented with different metals (Pb, As, Cd, Cu, Co and Cr) and their
tolerance related to soil contamination and enzyme activities.
This chapter can widen the knowledge about the pressure that soil microflora
experience under the direct effect of different metals.
Rhizobium
and soil enzyme
activities may be useful for the evaluation of agricultural soils pollution, which may be
used on the improvement of soil productivity or on the reclaim of contaminated soils.
1.
I
NTRODUCTION
Soil is an important natural resource that needs to be preserved and, if possible, its quality
and productive ability improved. Doran and Parkin (1994) defined soil quality as the capacity
to function within an ecosystem and sustain biological productivity, maintain environmental
quality and promote plant, animal and human health. Soil biological and biochemical
properties are responsive to small changes that occur in soil properties, thereby providing
immediate and accurate information on changes in soil quality. For this reason, soil microbial
activity has a direct influence in the ecosystem stability and in soil fertility (Smith and
Papendick 1993). Microorganisms play a fundamental role on the biogeochemical cycles and
are involved in forming the structure of a soil (Harris and Birch, 1989). The physical,
chemical, and biological properties of the soil are all important for its behaviour (Arshad and
Coen, 1992; Parr et al., 1992). Characterization of this behaviour should focus on the
properties that are most sensitive to environmental stress (Dick and Gupta 1994; Elliott, 1997;
Pankhurst et al., 1995; Vanhala and Ahtiainen 1994).
Soil equilibrium can easily be disturbed, especially by human intervention. Heavy metal
contamination in soil is of major environmental concern on a world scale. Beside their natural
occurrence, heavy metals may enter the ecological environment through anthropogenic
activities, such as mining, smelting, sewage sludge disposal, application of pesticides and
inorganic fertilizers and atmospheric deposition (Alloway, 1995a; Carrasco et al., 2005; Giller
et al., 1989; Malik, 2004; McGrath et al., 1995; Robinson et al. 2001; Shen et al. 2005).
Since microorganisms are the most abundant and most genetically diverse living
organisms adapted to almost all environments that exist on the Earth, they have been object of
study in numerous works on environmental stress. Several authors have shown that metals
adversely influence microorganisms (Shi et al., 2002), affecting their growth, morphology
and activities (Bååth et al., 1998; Lakzian et al., 2002; Pereira et al., 2006;), including
symbiotic N
2
fixation (Castro et al., 1997; Chaudhary et al., 2004). It is well known that
metals are direct and/or indirectly involved in all aspects of microbial growth, metabolism
and differentiation. Some metals such as Zn, Cu, Ni and Cr
are essential or beneficial
